Rotary piston pump

ABSTRACT

A pump having a biarcuate chamber closed by plane parallel walls, an eccentrically driven piston located in the chamber, a guide arranged between each side of the piston and a cylindrical surface of the chamber in order to translate the rotary drive to the piston into a reciprocal motion. Supply and delivery ducts arranged to open into the chamber from one of the plane parallel walls and associated recesses provided in the piston and the guides, whereby liquid to be pumped is supplied to and from a pump chamber through the selected opening and closing of slots provided in the other plane parallel wall so that each duct is placed in communication with the pump chamber by way of the associated recess in the piston.

United States Patent Inventor Appl. No.

Filed Patented Assignee Priority Knapp Mikrohydnulik G.m.b.H.

Neutraubling, Germany Oct. 31, 1968 Germany ROTARY PISTON PUMP 13 Claims, 6 Drawing Figs.

US. Cl, Int. Cl

Field of Search References Cited UNITED STATES PATENTS 1/1938 Fourness 2,221,149 11/1940 Potter 2,767,660 10/1956 Baler 417/465 3,242,869 3/1966 Komhyr 417/465 FOREIGN PATENTS 376,882 7/1932 Great Britain 417/465 655,404 7/1951 Great Britain 417/465 Primary Examiner-Carlton R. Croyle Assistant Examiner-wilbur .I. Goodlin Attorney-Jacobi, Davidson & Kleeman ABSTRACT: A pump having a biarcuate chamber closed by plane parallel walls, an eccentrically driven piston located in the chamber, a guide arranged between each side of the piston and a cylindrical surface of the chamber in order to translate the rotary drive to the piston into a reciprocal motion. Supply and delivery ducts arranged to open into the chamber from one of the plane parallel walls and associated recesses provided in the piston and the guides, whereby liquid to be pumped is supplied to andfrom a pump chamber through the selected opening and closing of slots provided in the other plane parallel wall so that each duct is placed in communication with the pump chamber by way of the associated recess in the piston.

PATENTEUJUL sum 3,591,318

sum 1 or 2 ROTARY PISTON PUMP The invention relates to a fully reversible rotary piston pump.

Rotary piston pumps are known which can be driven in one pumping direction only, other pump constructions are known, by which means of which continuous operation under fully identical pumping conditions (pressure and quantity of the material pumped) is possible selectively in both possible pumping directions simply by reversal of the directions of rotation of the eccentric, and which may be termed fully reversible rotary piston pumps. All these constructions have an almost fully cylindrical, eccentric-driven annular piston which is guided in a rotatable support by means of a narrow, slidable guide part, called a slide tongue. Owing to these dimensional relationships, large pump piston displacement spaces are certainly possible but high driving speeds and sealing pressures between the peripheral surfaces of the annular piston and pump housing are prohibited on account of the low mechanical load capacity of the connection between the annular piston and tongue, regardless of whether annular piston and tongue are made in one piece in a manner, which is expensive from the point of view of production engineering, or are constructed in two pieces connected together by a fitted joint. The narrow supporting area between the tongue and the end faces of the pump housing requires corresponding bearing pressures for the necessary axial sealing, which result in high frictional resistances and hence additional driving power and heavy wear, especially at high driving speeds. Axial seals between the annular piston and the end faces of the pump housing are provided in all cases by means of resiliently pressure-loaded slide rings, which in turn owing to the relatively small bearing surfaces and hence specifically high pressure loading, produce friction losses and wear phenomena. In addi tion, the transitions between the bearing surface of the annular piston and that of the tongue give rise to constructional difficulties.

The narrow tongue also has a further unfavorable effect because, in the known constructions, the connecting conduits for the pumped material, opening in the region of the rotatable supports, have to be connected to the pump chambers by means of two separate openings machined in the lateral walls of the tongue, which walls are formed as slide valves. Owing to the tongue dimensions, these openings naturally have only a small flow cross section compared with the piston displacement volume, which leads to high flow losses and hence a low volumetric efficiency at high pumping speeds.

Another disadvantage is also to be seen in the fact that the chamber which is formed between the guide part of the housing and the end of the tongue remote from the annular piston, and which, owing to the longitudinal movement of the tongue, acts as variable volume pump chamber, is not provided with any intentional inflow and outflow, so that the unavoidably entering leakage liquid rapidly forms during build up of the working pressure, a pressure cushion directed against the tongue movement. The forces necessary for overcoming this pressure cushion can considerably impair the efficiency of the pump. While in the case of open liquid circulation systems, this disadvantage may be met in a relatively simple manner, although technically not entirely satisfactorily, by the provision of an outlet from the pump chamber to the outside, and a return to the liquid supply vessel, such a step cannot be used in systems with closed liquid circulation, which have to be operated for as long as possible without being filled up with liquid.

It is an object of the present invention to provide a pump in which the intentional flow of liquid through a pump chamber is possible without having to lead liquid to the outside from this chamber.

It is a further object of the invention to provide a rotary piston pump which is suitable for high speeds of rotation and wide flow connections between connection conduits and pump spaces, while obviating the above-mentioned disadvantages. In particular, for systems with closed liquid circulation, leakage losses of the pump are to be completely intercepted and resupplied to the circuit, in order thereby to permit a long service period without maintenance.

According to a first aspect of the invention there is provided a rotary piston pump comprising a housing defining an inner space, an elongate piston located in the inner space, an eccentric drive shaft connected to the piston at a position adjacent to, but spaced from, one end of the piston, rockable piston guides located in the inner space with their facing surfaces in slidable contact with the piston, a piston chamber formed in part by a sidewall of the piston opposite said one end and the facing surfaces of the guides, two ducts serving respectively as pressure and supply conduits opening into the inner space at a position intermediate the sidewall and said one end of the piston, two spaced-apart recesses in the piston opposite the ducts, said recesses being spaced from the sidewall, and two connections, one associated with each duct, disposed to open into the pump chamber and which in use are selectively opened and closed by the displacement of the piston to place the piston chamber in communication with either one of the ducts by way of its associated recess.

According to a second aspect of the invention there is provided a rotary piston pump comprising a housing defining therein an inner space including a part-cylindrical section, two plane parallel end walls mounted in a fluidtight fashion one on each side of the housing, an elongate piston comprising a semicylindrical part and a rectilinear-shaped part, mounted to be slidable sealingly between the end walls so that the semicylindrical part of the piston is slidable on the part-cylindrical section of the inner space; an eccentrically mounted drive shaft connected to the semicylindrical part of the piston, two rockably mounted guide members arranged to be in slida' ble sealingly engagement with respective sides of the rectilinear-shaped part of the piston, two spaced-apart ducts opening into the inner space in a region of the rectilinearshaped part of the piston, said ducts serving, respectively, as a pressure conduit and a suction conduit, two spaced-apart recesses in the rectilinear-shaped part of the piston, each of said recesses opening into an associated one of the two ducts, a pump chamber formed in part in the inner space between the free end of the piston, the end walls and the guide members, and two connections, one associated with each duct, disposed to open in the pump chamber and which in use are selectively opened and closed by the rectilinear-shaped part of the piston being displaced by rotation of the drive shaft to provide a fluid path between each duct and the piston chamber via the associated piston recess.

A pump made in accordance with the invention has the advantage of an extremely high mechanical loading capacity, together with a particularly simple form from the production engineering standpoint. in addition, the piston, which does not have special sealing means, bears with the larger part of its end faces against the inner chamber end faces of the housing, thereby providing good axial scaling for only a slight specific surface pressure. In this way, high driving speeds are possible without overloading the piston mechanically, and/or appreciable wear. This particularly large bearing surface even permits temporary nondestructive driving of the pump in the event of interruption in the supply of liquid. Thus, with a considerably increased working safety and life, a saving in special axial-sealing constructions is also achieved.

It is furthermore possible for the recesses provided in the rectilinearly shaped part of the piston and which is necessary for connecting the ducts to the piston displacement spaces to be made many times larger than was the case with the tongues of previous constructions, in proportion to the piston displacement space. The resistance to flow is thereby considerably reduced, whereby higher pumping speeds and delivery quantities at a greater volumetric efficiency are possible. In a preferred embodiment, the connecting ducts in the housing end face open above the rotary piston recesses which are open towards this side. Contrary to hitherto existing constructions,

in which the ducts open in the cylindrical sidewalls of the guide housing and are connected by openings in the guide pieces to the recesses in the tongue, the preferred arrangement of the ducts is independent of the axial dimension of the rotary piston. In the case where channels are led in the pump axis direction, such as is frequently the case in a space-saving arrangement of the pump together with other elements, the end-side opening eliminates a 90 deflection of the flow. Another advantage of the preferred channel arrangement resides in the particularly simple guide-piece construction thus made possible.

As already known from the prior art, the guide pieces may cooperate with the rotary piston recesses, taking into consideration longitudinal sliding of the piston, such that in the zero delivery position of the piston, the ducts serving respectively as pressure conduit and suction conduit are isolated from each other. This slide valve control, however, calls for high production accuracy and in addition greatly constricts the otherwise possible effective flow cross section of the recesses. Particularly in systems in which, for example, for hydraulic locking of a piston drive included in the delivery circuit, nonrcturn valves are provided in any case, such constriction of the ducts within the pump itself will therefore be omitted. In systems with open liquid circulation a two-way valve control is used for this purpose, while in systems with closed liquid circuit, a double-acting, hydraulically precontrolled nonreturn valve can be used.

An important feature of the invention is in the intentional flow of liquid through the pump chamber formed between the rear end of the slidable piston part and the rear part of the guide housing. According to the invention, by means of the piston movement, at least during part of the suction stroke, communication is established from the pump chamber to the suction space, thereby preventing build up of a liquid back pressure and hence a pressure cushion in this chamber. Such a control is obviously also possible in the case of pumps, which can be used for only one delivery direction. In fully reversible pumps, all the constructions on the rotary piston and pump housing must be formed with mirror-image symmetry relative to the longitudinal plane of symmetry perpendicular to the end faces. A preferred arrangement of the connections in relation to the different piston positions permits a type of positively controlled four-stroke control, which will be explained more fully with reference to the embodiment example described later. It should be emphasized that the preferred formation of the connection as slots and their provisiorgjn the end face plane of the housing is not obligatory.

In a particularly preferred embodiment of the solution according to the invention, the pump chamber is additionally connected to the bearing space of the eccentric shaft, to which the leakage liquid flows from the eccentric pin. Interposition of a nonretum valve ensures that no liquid can flow back from the pump chamber to the bearingspace. On the other hand, during the suction stroke, liquid can flow away from the bear ing space to the pump chamber, so that no appreciable pressure builds up in the bearing space and relatively simple sealing means may be used for the drive unit. It should be emphasized that the suction exerted on the bearing space is advantageously much weaker than the suction in the pump suction space, so that leakage liquid flow will not be unnecessarily increased and consume power. On the contrary, pressure build up is prevented only in the bearing space (as in the pump chamber). It is easy to see that this step is particularly valuable in systems with closed liquid circulation, such as are used for example as hydraulic microdrives, since no loss of liquid can occur at the pump itself by the return of all leakage liquid into the circuit.

It should also be pointed out that the piston displacement spaces, small in themselves with regard to the cross-sectional dimensions, can by correspondingly larger axial dimensions be adapted immediately to the displacement spaces of known piston constructions. On the other hand, the quantity delivered, by reason of high drive speed and the better volumetric efficiency through substantially smaller flow resistances even for small displacement spaces, may be increased quite considerably, the advantage of a low-amplitude high frequency pulsation being obtained. Resonance phenomena with the natural frequencies of othercomponents present in or coupled to the delivery circuit are more readily avoided, or any equalizing devices provided for the absorption of the delivery pulses may be made smaller.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 shows a sectional view along the line ll of FIG. 2 through an embodiment of a pump made in accordance with the invention;

FIG. 2 shows an elevational view of the pump shown in FIG. 1 with the housing open; and

FIGS. 3a 3d show elevational views corresponding to FIG. 2 but with the rotary drive of the piston displaced through four successive quadrents.

FIGS. I and 2 show the sandwich-type of construction of the pump made in accordance with the invention. An inner space of a pump housing 1 consists of two, partially cylindrical sections 2 and 3 of different diameters arranged directly adjacent each other, and it is thereforeeasy to make. The axial end faces 4 and 5 of the inner space of the housing are formed by the surfaces of adjoining parts 6 and 8, of which one part 6 serves for accommodating a shaft 7 driving an eccentric and the other part 8 for accommodating any components, not shown, following the pump, for example a double-acting, hydraulically precontrolled nonretum valve connected to two ducts 9 and 10.

A piston 11, mounted in the inner space 2,3 is made in one piece and comprises a semicylindrical shaped piston part 12 and a rectilinear piston part 15 mounted so as to be longitudinally slidable between two guide pieces 13 and 14, arranged one on each side of the part 15. An eccentric in the form of a pin 16 is arranged with its axis concentric with the axis of the semicylindrical-shaped piston part 12, the pin 16 is arranged offcenter by a suitable amount from axis parallel to the pumpside end of the shaft 7. A sleeve 17 is rotatably mounted on the eccentric pin 16. The sleeve 17 has two axially spaced peripheral grooves 18, in which are mounted 0 rings 19 serving as resilient elements. Engaging the radial surface of these 0 rings under slight prestressing is the wall of a corresponding bore 20 of the piston 11, the radii of the cylinder walls of the piston part 12 and of the housing section 2 being selected such that the latter bear linearly against each other under predetermined prestressing and slide along on each other in the driven condition.

In positions corresponding to the mouths of the ducts 9 and 10, in the axial end face 5, two recesses 21 and 22 which open towards the end face 5, are provided in the piston 11, so that each duct opens into a respective recess of the piston. These recesses 21, 22 are open towards the longitudinally guided piston sidewalls and open in recesses 23 and 24, respectively, provided in the respective guide pieces 13 and 14, which form the flow connections from the respective ducts 9 and 10 to respective piston displacement spaces 25 and 26 (FIG. 3) formed during operation. The recesses 21, 22, 23 and 24 of the rotary piston 11 and of the guide pieces 13 and 14 are equally deep axially but do not extend over the entire axial dimension of the piston, thereby leaving long guide surfaces between the guide pieces and the piston in the piston region adjacent the end face 4.

A pump chamber 29 is provided which is confined radially between a piston side 27 remote from the semicylindrically or U-shaped part 12 and a cylindrical periphery side 28. The chamber 29 thus represents a part of the partly cylindrical housing section 3. Two slots 30, 31 are machined in the end face 4, the slots are disposed as mirror-images symmetrically of the longitudinal plane through the end face 4. Preferably, the slots have a cross section of circular segment shape which is advantageous from the production engineering standpoint.

According to the working position of the piston, these slots 30 and 31 are connected respectively to bores 32 and 33, provided in the bottoms of the two recesses 21 and 22.

This embodiment example shows a connection 36, provided more particularly in systems with closed liquid circulation, between the pump chamber 29 and a bearing space 34, in which is inserted a nonretum valve 35. The entire mounting of the driving shaft 7 and the arrangement of the connection 36 with the interposed nonretum valve 35 in the part 6 are shown diagrammatically, since normal components are concerned. As will be seen from the view in FIG. 1, no special sealing means is provided between the eccentric pin 16 and the driving shaft 7, so that any leakage liquid issuing between the end faces 4 and 5 of the pump housing and those of the piston 11, which glide on one another without special sealing arrangement, can pass unimpeded to the bearing space 34. The nonretum valve 35 prevents flow of liquid from the pump chamber 29 to the bearing space 34, but permits suction of liquid from the bearing space 34 into the pump chamber 29 under a slight suction effect, possibly supported by a low pressure in the bearing space 34. Due to the fact that considerable pressure cannot build up in the bearing space, the symbolically shown axial and radial seal 37, which seals off the bearing space 34 towards the side of the driving unit, not shown, is very simple and cheap to construct.

It should be noted that the pump chamber 29, on account of the variation in volume taking place rhythmically in it, may also be utilized as pump chamber effective for delivery, with the possible assistance of a special check valve arrangement.

Referring to FIGS. 30 to 3d the mode of operation of the pump made in accordance with the invention will be explained. FIG. 3a shows the so-called zero delivery position of the piston 11, in which the two ducts 9 and 10, sewing selectively as suction conduit or pressure conduit, are connected together. Blocking of the circuit in this piston position is effected by double-acting, hydraulically precontrolled nonretum valves, not shown. In this piston position, the slots 30 and 31 are closed by the sliding over of the piston 11 towards the pump chamber 29, while the bores 32 and 33 are connected to the respectively associated slots.

In FIG. 3b the piston position is shown after at 90 rotation of the eccentric driving shaft 7. Until this position is reached, liquid has been forced into the pump chamber 29 through the slot 30, so that the pump movement is vigorously supported. It should, however, be emphasized that the slot cross sections are small in proportion to the cross sections of the delivery circuit, so that in the respective time of the working stage, owing to the piston movement speeds, complete pressure equalization does not occur. When the piston position shown in FIG. 3b is reached, the slot 30 is closed by displacement of the bore 32, whereas the slot 31 hasjust opened by way of the bore 33. In this stage, the suction stroke of the pump begins and is thus propagated into the pump chamber 29.

FIG. 30 after a further 90 rotation, shows the piston in the position of maximum delivery performance. In this position, the slots 30 and 31 have been reclosed by displacement of the bores 32 and 33; at the same time, the pump chamber has reached its maximum volume, and can in this position draw liquid from the bearing space 34 through the nonreturn valve 35.

FIG. 3d shows the rotary piston in the position after a further 90 of rotation, i.e. 270 in all, in which the pump exerts its maximum pressure. Due to the displacement of the piston, the mouth of the slot 30 is only very slightly opened by the sidewall 27 of the piston, while the slot 31 has been closed by a corresponding displacement of the bore 33. The influx of liquid during further rotation of the piston is slight, since at the commencement of the zero delivery stage, the nonretum valve closes the pressure conduit. On the contrary, during the compression stroke in this stage, liquid can escape through the two slots from the pump chamber 29, so that no appreciable pressure cushion can form against the piston movement. On reaching the zero delivery position (FIG. 3a the slots 30 and 31 are closed again.

This mode of operation, divisible into four stages, of the socalled positive four-stroke slot control, shows that build up of a pressure cushion in the pump chamber 29 is reliably prevented; quite on the contrary the suction effect obtained can be utilized for relieving the load of spaces affected by leakage liquid. Measurements have led! to the surprising result that such a pump in microconstruction at a no-load speed of approximately 8,000 r.p.m. can build up pressures of the order of magnitude of forty atmospheres above atmospheric pressure.

The invention is not limited to the following claims but covers all the features following from the description and drawings which are manifestly inventive in view of the prior art.

lclaim:

l. A rotary piston pump comprising a housing defining therein an inner space including a part cylindrical section, two plane parallel walls to said housing; a piston sealingly slidable between parallel walls; faces to the piston slidable engaging said parallel walls; a semicylindricall'y shaped end to said piston slidable on the part cylindrical section of the inner space; a rectilinear-shape part to said piston; a second end to the piston remote from the semicylindrically shaped end; an

eccentrically mounted drive shaft connected to the semieylindrically shaped end of the piston; two guide members roekably mounted with respect to the housing, the rectilinear-shaped part of the piston, being sealingly slidable between said guide members; two spaced-apart ducts opening into the inner space in the region of the rectilinear-shaped part of the piston, one of said two ducts serving as a pressure conduit and the other of said two ducts serving as a suction conduit for liquid to be pumped; two spaced-apart recesses opening into one face of the piston in positions adjacent to the ducts; a pump chamber formed in part in the inner space between said second end, the guide members and the parallel walls; and two connections, one associated with each duct and opening into the pump chamber and selectively openable and closable by the rectilinear-shaped part of the piston effective to provide a fluid path between each duct and the piston chamber via the associated piston recess.

2. A rotary piston pump as claimed in claim ll, further comprising another part cylindrical section of the inner space arranged axially parallel with the first-mentioned part cylindri cal section.

3. A rotary piston pump as claimed in claim 2, in which the two guide members have a part cylindrical section of a similar radius of curvature as the another part cylindrical section, said guide members being slidable sealingly on the another part cylindrical section of the housing about the axis thereof.

4. A rotary piston pump as claimed in. claim 1, further comprising a recess in each guide member, which recess communicates with an associated duct and piston recess, and piston displacement spaces located in the part cylindrical section of the inner space, one on each side of the piston, each piston displacement space being in communication with the recess in an associated guide member.

5. A rotary piston pump as claimed in claim 1, further comprising a bearing space for the drive shaft in one of the walls, a conduit interconnecting the bearing space and the pump chamber, and a nonreturn valve positioned in the conduit effective to permit the flow of liquid to be from the bearing space to the pump chamber only.

6. A rotary piston pump as claimed in claim 1, in which the piston contacts directly said parallel walls.

7. A rotary piston pump as claimed in claim 1, in which the recesses in the piston open towards the guide pieces and to one face of the piston, and further comprising a web extending between said recess in the axis of symmetry of the piston to separate the recesses.

8. A rotary piston pump as claimed in claim 7, in which the ducts open into that parallel wall facing said one face of the piston.

9. A rotary piston pump as claimed in claim 7, in which said two connections comprise slots provided in said other parallel wall said slots being in mirror-image symmetrically to the plane of symmetry of the housing extending perpendicularly to the parallel walls.

10, A rotary piston pump as claimed in claim 8, further comprising two bores in the piston, said bores opening respectively into one of the piston recesses.

11. A rotary piston pump as claimed in claim 10, in which the diameter of the bores are equal to each other, said diameter and the eccentricity of the drive shaft are selected so that in the zero delivery position of the piston, in which the volume of the pump chamber is a minimum, both the connections between the pump chamber and the ducts are closed; in the position of the piston after rotation of the eccentric shaft by 90 from the zero delivery position, the connection leading to the duct acting as suction conduit is opened, and the other connection leading to the duct acting as pressure conduit is closed; in the position of the piston after a further rotation of the eccentric drive shaft both said connections are again closed and in the position of the rotary piston after another 90 rotation of the eccentric shaft the connection associated with the duct acting as pressure conduit or with the pressure space is opened, and the other connection associated with the duct acting as suction conduit is closed.

12. A rotary piston pump as claimed in claim 1, further comprising at least one resilient sealing element provided between the eccentric drive shaft and the piston.

13. A rotary piston pump as claimed in claim 3, in which said another part cylindrical section of the inner space is arranged directly adjacent the part cylindrical section of the mner space. 

1. A rotary piston pump comprising a housing defining therein an inner space including a part cylindrical section, two plane parallel walls to said housing; a piston sealingly slidable between parallel walls; faces to the piston slidable engaging said parallel walls; a semicylindrically shaped end to said piston slidable on the part cylindrical section of the inner space; a rectilinear-shape part to said piston; a second end to the piston remote from the semicylindrically shaped end; an eccentrically mounted drive shaft connected to the semicylindrically shaped end of the piston; two guide members rockably mounted with resPect to the housing, the rectilinearshaped part of the piston, being sealingly slidable between said guide members; two spaced-apart ducts opening into the inner space in the region of the rectilinear-shaped part of the piston, one of said two ducts serving as a pressure conduit and the other of said two ducts serving as a suction conduit for liquid to be pumped; two spaced-apart recesses opening into one face of the piston in positions adjacent to the ducts; a pump chamber formed in part in the inner space between said second end, the guide members and the parallel walls; and two connections, one associated with each duct and opening into the pump chamber and selectively openable and closable by the rectilinear-shaped part of the piston effective to provide a fluid path between each duct and the piston chamber via the associated piston recess.
 2. A rotary piston pump as claimed in claim 1, further comprising another part cylindrical section of the inner space arranged axially parallel with the first-mentioned part cylindrical section.
 3. A rotary piston pump as claimed in claim 2, in which the two guide members have a part cylindrical section of a similar radius of curvature as the another part cylindrical section, said guide members being slidable sealingly on the another part cylindrical section of the housing about the axis thereof.
 4. A rotary piston pump as claimed in claim 1, further comprising a recess in each guide member, which recess communicates with an associated duct and piston recess, and piston displacement spaces located in the part cylindrical section of the inner space, one on each side of the piston, each piston displacement space being in communication with the recess in an associated guide member.
 5. A rotary piston pump as claimed in claim 1, further comprising a bearing space for the drive shaft in one of the walls, a conduit interconnecting the bearing space and the pump chamber, and a nonreturn valve positioned in the conduit effective to permit the flow of liquid to be from the bearing space to the pump chamber only.
 6. A rotary piston pump as claimed in claim 1, in which the piston contacts directly said parallel walls.
 7. A rotary piston pump as claimed in claim 1, in which the recesses in the piston open towards the guide pieces and to one face of the piston, and further comprising a web extending between said recess in the axis of symmetry of the piston to separate the recesses.
 8. A rotary piston pump as claimed in claim 7, in which the ducts open into that parallel wall facing said one face of the piston.
 9. A rotary piston pump as claimed in claim 7, in which said two connections comprise slots provided in said other parallel wall said slots being in mirror-image symmetrically to the plane of symmetry of the housing extending perpendicularly to the parallel walls.
 10. A rotary piston pump as claimed in claim 8, further comprising two bores in the piston, said bores opening respectively into one of the piston recesses.
 11. A rotary piston pump as claimed in claim 10, in which the diameter of the bores are equal to each other, said diameter and the eccentricity of the drive shaft are selected so that in the zero delivery position of the piston, in which the volume of the pump chamber is a minimum, both the connections between the pump chamber and the ducts are closed; in the position of the piston after rotation of the eccentric shaft by 90* from the zero delivery position, the connection leading to the duct acting as suction conduit is opened, and the other connection leading to the duct acting as pressure conduit is closed; in the position of the piston after a further 90* rotation of the eccentric drive shaft both said connections are again closed; and in the position of the rotary piston after another 90* rotation of the eccentric shaft the connection associated with the duct acting as pressure conduit or with the pressure space is opened, and the other connection associated wIth the duct acting as suction conduit is closed.
 12. A rotary piston pump as claimed in claim 1, further comprising at least one resilient sealing element provided between the eccentric drive shaft and the piston.
 13. A rotary piston pump as claimed in claim 3, in which said another part cylindrical section of the inner space is arranged directly adjacent the part cylindrical section of the inner space. 